docs: Replaced SqliteSaver with MemorySaver and updated installation instru… (#25285)

…ctions to match LangGraph v2 documentation. Corrected code snippet to
prevent validation errors.

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- [ ] **PR title**: `docs: update checkpointer example in Conversational
RAG tutorial`

- [ ] **PR message**:
- **Description:** Updated the Conversational RAG tutorial to correct
the checkpointer example by replacing `SqliteSaver` with `MemorySaver`.
Added installation instructions for `langgraph-checkpoint-memory` to
match LangGraph v2 documentation and prevent validation errors.
    - **Issue:** N/A
    - **Dependencies:** `langgraph-checkpoint-memory`
    - **Twitter handle:** N/A

- [ ] **Add tests and docs**: 
  1. No new integration tests are required.
  2. Updated documentation in the Conversational RAG tutorial.

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docs/docs/tutorials/qa_chat_history.ipynb

@@ -857,9 +857,9 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "from langgraph.checkpoint.sqlite import SqliteSaver\n",
+    "from langgraph.checkpoint.memory import MemorySaver\n",
     "\n",
-    "memory = SqliteSaver.from_conn_string(\":memory:\")\n",
+    "memory = MemorySaver()\n",
     "\n",
     "agent_executor = create_react_agent(llm, tools, checkpointer=memory)"
    ]
@@ -1022,10 +1022,10 @@
     "from langchain_core.runnables.history import RunnableWithMessageHistory\n",
     "from langchain_openai import ChatOpenAI, OpenAIEmbeddings\n",
     "from langchain_text_splitters import RecursiveCharacterTextSplitter\n",
-    "from langgraph.checkpoint.sqlite import SqliteSaver\n",
+    "from langgraph.checkpoint.memory import MemorySaver\n",
     "from langgraph.prebuilt import create_react_agent\n",
     "\n",
-    "memory = SqliteSaver.from_conn_string(\":memory:\")\n",
+    "memory = MemorySaver()\n",
     "llm = ChatOpenAI(model=\"gpt-3.5-turbo\", temperature=0)\n",
     "\n",
     "\n",